Willows: choking pools or creating them?

Willows are said by some to ‘choke’ pools and streams: check here, here and here.

Apparently that’s one of the reasons they’re so terribly bad for platypus; like the one in this video.

Then again, the platypus pool in this video is actually created and maintained by a bank of willows, you can see some of them in the background. These have colonised and aggraded the ‘floodout’ of deposited sediment at the pool’s downstream end (shown in the clip). The pool is approximately 150m long, averaging 4-5m across and is about 160cm at its deepest point. It lies within an incised section of Monkittee Creek which is well-lined with willows. Platypus are regularly sighted in this pool, sometimes two at a time.

Below is a short video of another willow which is holding back a pool of water and creating a stable riffle within the incision on Gilamatong Creek (on the Braidwood Common). This pool is about 50m long, although quite narrow (about 1.5m wide for the most part) as it conforms to the width of the incision.

Here is a still image of this same willow-riffle. You can see young culms (it’s only October) of Phragmites australis growing up out of the root mat at the top and alongside of the riffle. You can also see buttercups and an unidentified grass species (center right) colonising the willow root mat. These are growing in what was once pure root mat, but is fast becoming a mix of decomposed roots and trapped sediments (i.e. a fertile and hydrated soil medium).

willow riffle

Here is another pool created and stabilised by a willow.

This pool is slightly below the one shown in the video above. A series of willow ponds form a sequence of steps within the incision along this section of the creek. At each step a willow and its root mat is holding back a pool of water and creating a riffle (like in the first video) or a plunge pool with multiple overflow pathways (like in the second).

The downstream plunge pool produces an effect referred to as ‘water on water’ (a phenomenon which lessens erosive stream energy). It acts much like the ‘stilling ponds’ installed at the base of cement flumes and other expensive engineered structures.

In function, the series of willow-pools within this incised flow-line resemble the ‘chains of ponds’ that would naturally have occurred within many southeast Australian swampy meadow floodplains prior to the arrival of European grazing domesticates (which caused widespread pond incision). Without these stepped pools, this incision would be a gouged-out flat-bottomed race, like many other eroded flow-lines.

The willow-created pools introduce significant structural diversity into the incision. Without them, base-flows proceeding along a more or less fixed incline, from upstream to down, would contain more erosive stream-energy. With them, water is slowed within the pools and sediments are likely to settle out. A functional pool-riffle stream morphology emerges. A permanent store of standing water within the pools is maintained even during low flow periods. This enables the establishment of common native macrophytes like Typha, Baumea and Phragmites.

In these widespread incised flow-line situations there would often be no pools to “choke” without the presence of these stabilising willows.

So maybe, just maybe, we need to reconsider those simple-minded assertions about willows choking streams and pools.

I’m not saying it doesn’t happen, but all aquatic plants will colonise  and “choke” pools and streams given the right conditions. Typha, Baumea and Phragmites all do it too. In fact, this is a perfectly natural and desirable process.  I’ll write shortly about why this is the case, and explain why this “choking” has always been a part of the Ecogeomorphology of Australian landscapes. Endemic swampy meadows and chains of ponds are a perfect testament to this natural fluvial process.

However, for this post, all that is needed is to suggest that perhaps there is more to understanding landscape function than can be conveyed by a list of “good” and “bad” species. Perhaps genetic ‘purity’ within a landscape cannot directly relate to its health, stability and resilience following disturbance.

Perhaps the widespread and uncompromising demonisation of the willow is no longer really tenable and one day it will start to seem extremely counterproductive and silly.

Time for a another look?

4 responses to “Willows: choking pools or creating them?

  1. Hi Ben, I’ve enjoyed checking out your writing this evening.

    I just got back from a week long stream management course down in Vic at which the unquestioned nativist ideology was certainly strong. You know things are extreme when even Casuarina cunninghamiana is shunned from bank stabilisation attempts, despite excellent council-planted specimens existing only 50 metres away, all because a strong Northerly didn’t blow some of their papery seed over a couple of hills a few hundred years ago. In confusion, while observing the gold standard examples we were shown, I was instead looking at an Acacia dealbata dominated stand which had about 8 years to go before the lot fell over and needed planting again.

    Granted, the removed willows weren’t appropriate in this instance as they increased the likelihood of flooding the poorly located local town, but surely if the site warrants log piles (along with excavator damage after 15 years to replace them once they rot), to prevent the scour and erosion of an adjacent potato plot, a tree which is a bioengineering champion from only 100km away could be thrown a bit of leeway. Nope, January 26, 1788 was the day that humans hit pause on Nature.

    Keep up the good work, Cam

  2. Cam ,

    Can you tell us more about the course ?
    What did you learn , what did you teach .? Any photos ?
    Useful insights , course notes etc .

    Social dynamics of a week spent with nice folk nativists ?

    Regards

    PM

  3. Hi Pete,

    In short it was a number of the top academics and professionals related to stream ecology, hydrology, geomorphology and restoration from Victoria and the MDB providing a snapshot of stream assessment and designing and prioritising rehabilitation pathways in the context of an ever decreasing budget (Pretty much plants over engineering and putting back large woody debris is very fashionable). The course programme can be seen here: http://www.wisewaterways.org.au/documents/2014WiseWaterWaysProgram_000.pdf

    I used it as a great networking opportunity, and took each presenter aside after their talk to get their take on upper catchment processes, focusing on swampy meadow restoration in particular. There was plenty of support from people in each discipline and I got some good contacts and leads on a few of the major sticking points from a regulatory point of view.

    Not too many new insights but a couple of extremely well backed approaches in the lower catchments which we could definitely borrow wording and precedents from include the well-accepted ecological benefits from reinstating floodplain connectivity and the millions being spent on re-snagging streams to restore lost habitat as an analogue for reedbeds, back swamps and backwaters in our neck of the woods.

    Gotta go but happy to run through more in person.

    Cheers, Cam

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